Reusable shopping bags are green, but when shoppers don't wash them, the bags become a breeding ground for harmful bacteria.

It is the same type of cross-contamination that happens when the same utensil is used for uncooked and cooked food, said Dr. Jane Huffman, professor of biological sciences at East Stroudsburg University.

The University of Arizona and Loma Linda University conducted a 2011 study to learn how much and what type of bacteria could be found in reusable grocery bags.

Shoppers may be surprised by what the researchers discovered.

Testing 84 reusable grocery bags for bacteria, researchers found all the bags but one showed high levels of bacteria, more than half tested positive for coliform — usually associated with raw meat or uncooked food — and 12 percent of the bags tested positive for E. coli.

"I classify them as pretty dirty things, like the bottom of your shoes," said Ryan Sinclair of the Loma Linda University School of Public Health and co-author of the study.

He said the contamination cycle often begins after shoppers enter the store and place their bags in the bottom or the baby carrier of a shopping cart.

Sinclair is working on some new data to show pathogen movement throughout a grocery store.

The new study, conducted at a central California grocery store in early 2013, involved spraying bags with a bacteria not harmful to humans but transported in a similar way to norovirus, a leading cause of gastrointestinal disease linked to more than 19 million illnesses each year in the U.S. The results haven't been published yet.

"You will never be able to eliminate all the bad bacteria, but you can take precautions to keep the level of harmful bacteria low enough to be safe," Huffman said.

Using bags that previously held groceries to carry other things, such as snacks or books, can transfer harmful bacteria.

In 2012, an outbreak of the norovirus among a girls' soccer team from Oregon was traced to a reusable bag stored in a hotel bathroom.

Norovirus causes about 21 million illnesses, 70,000 hospitalizations and 800 deaths a year in the U.S., according to the CDC.

Even if the bag hasn't been used for a while, some types of bacteria can still be present, Huffman said.

When conditions change and water and food become unavailable, some strains of bacteria have the ability to go dormant and survive for weeks or even years by turning into a spore.

"The spores are able to survive through droughts, heat and even radiation," Huffman said.

"When more water or food becomes available, the bacteria 'comes to life' again and transforms from a spore back into a live cell," she said.